Organometallic iridium catalysts based on pyridinecarboxylate ligands for the oxidative splitting of water

Organometallic compounds [Cp*Ir(κ²-N,O)X] (κ²-N,O = 2-pyridinecarboxylic acid, ion(−1) (1), 2,4-pyridinedicarboxylic acid, ion(−1) (2), 2,6-pyridinedicarboxylic acid, ion(−1) (3); X^– = Cl^– (a), NO₃– (b)) and [Ir(κ³-N,O,O)(1-κ-4,5-η²-C₈H₁₃)(MeOH)] (κ³-N,O,O = 2,6-pyridinedicarboxylic acid, ion(−2) (4)) are effective catalysts for the oxidative splitting of water to O2 driven by Ce⁴⁺. They show similar TOFLT values (long-term TOF, 2.6–7.4 min^–1) while TOFIN values (initial TOF) strongly depend on the catalyst (1 ≫ 2 > 3 > 4), reaching a maximum value of 287 min–1 (4.8 s^–1) for 1a, which is the highest TOF value ever reported for an iridium catalyst. Voltammetric measurements indicate that the oxidative processes of compounds 1–4 are located at values substantially less positive than that of [Cp*Ir(bzpy)NO₃] (bzpy = 2-benzoylpyridine; ΔE ≈ 0.2–0.3 V), taken as reference catalyst for water oxidation. In particular, compound 3, having a pendant −COOH moiety in close proximity to an iridium coordination site, as shown by the structure determined by single-crystal X-ray diffraction, exhibits several low-potential oxidation processes.